1. Field of the Invention
This invention relates to circuit interrupters having digital controls which sample the load current at selected sampling intervals. More particularly, it relates to such digitally controlled circuit interrupters which automatically adjust the sampling interval for 50 Hz and 60 Hz power systems.
2. Background of Information
Many circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and such, now have digital control systems. Such circuit interrupters digitize the current through the interrupter by sampling it at prescribed intervals. Typically, this sampling interval is fixed at the factory and selected for compatibility with the frequency, either 50 Hz or 60 Hz, of the distribution system with which the interrupter is to be used. With the globalization of markets it has become desirable to provide a circuit interrupter which is easily adaptable for use with either frequency.
U.S. patent application Ser. No. 08/072,376, filed on Jun. 7, 1993, for "Electrical Switching Apparatus with digital Trip Unit and Automatic Frequency Selection" discloses a circuit interrupter with a digital control which automatically sets the digital sampling rate for the 50 Hz or 60 Hz distribution system to which the circuit interrupter is connected. The control system squares up the ac current signal and counts the leading edges of the resultant pulses to generate a count of the zero crossings. If this count is above a reference count, selected as about midway between a count of zero crossings for a 50 Hz signal and a 60 Hz signal during a reference interval, the sampling interval is set for a 60 Hz source. The sampling interval is set to the 50 Hz value if the count is below the reference count. The selected interval is stored in RAM (random access memory) for use by the microprocessor of the digital control and is also stored in an EEPROM (electronically erasable programmable only memory) for use by the microprocessor on start-up following an interruption of control power (typically, the digital control is powered by current in the protected circuit, so that control power is lost and with it values stored in RAM when the circuit interrupter is turned off or trips).
The above automatic selection of the sampling interval works well for 50 Hz and 60 Hz systems in which the current waveform is uncorrupted. However, many loads today, such as for example inverter based speed controls for ac motors, generate harmonics in the load current. These harmonics generate additional zero crossings which produce a false count in the automatic frequency selector. The problem arises when the circuit interrupter is used with a 50 Hz system as the extra counts will falsely result in selection of the interval for a 60 Hz system. As the problem can be intermittent, the interval switches back and forth between that for 50 and 60 Hz. The repeated change in the sampling interval may not result in unacceptable inaccuracies in the protection functions; however, it will probably produce unacceptable power and energy calculations, if generated, as in some circuit interrupters.
Another problem created by random switching between the two intervals for digital sampling of the current is that EEPROMs can only be written over a limited number of times, such as for instance 10,000 times. The entire EEPROM can become inoperable when this number of rewrites is reached, thus disabling other features of the control system having parameters stored in the EEPROM. The digital control of the above mentioned circuit interrupter has a custom integrated circuit chip (IC) described in U.S. Pat. No. 5,270,898 which has a shunt regulated power supply incorporating a chopper. This chopper can create noise which generates a false count of zero crossings in the automatic interval selector of the digital control, and ultimately, this can also lead to excessive rewrites of the digital sampling interval in the EEPROM.
There is a need, therefore, for an improved circuit interrupter for use with either a 50 Hz or a 60 Hz power distribution system.
More particularly, there is a need for such a circuit interrupter with a digital control which automatically selects an appropriate digital sampling rate for the frequency of the connected power source, but which is insensitive to the extraneous zero crossings of a distorted load current waveform.
There is a further need for such a circuit interrupter which does not generate extraneous rewrites for an EEPROM used to store the selected digital sampling interval.
There is also a need for such a circuit interrupter which is protected from false operation if the interrupter has a shunt regulated power supply which incorporates a chopper.